1. Field of the Invention
The present invention relates to a substrate for a light emitting diode and a light emitting diode employing the same. More specifically, the present invention relates to a patterned substrate for a light emitting diode and a light emitting diode employing the same.
2. Discussion of the Background
GaN-based light emitting diodes (LEDs) have been developed and applied for about 10 years. The GaN-based LEDs have significantly changed LED technologies and currently employed in a variety of applications including full color LED displays, LED traffic lights, white LEDs and the like.
Recently, high-efficiency white LEDs have been expected to replace common fluorescent lamps. Particularly, the efficiency of white LED is approaching that of general fluorescent lamps. However, there is room for further improvement in the efficiency of LEDs, and thus, further efficiency improvement is continuously required.
Two major approaches have been attempted to improve the efficiency of LEDs. The first approach is to enhance the internal quantum efficiency determined by the crystal quality and the epitaxial layer structure, and the second approach is to increase the light extraction efficiency.
Since the internal quantum efficiency currently reaches 70˜80%, there is little room for further improvement of the internal quantum efficiency. However, there is room for improvement in the light extraction efficiency. For the improvement in light extraction efficiency, it is important to eliminate internal loss by employing a heat radiation structure and roughened surfaces.
Meanwhile, roughened surfaces are employed to prevent total reflection due to a difference between refractive indexes of a GaN-based LED and the surrounding thereof, e.g. a substrate and the atmosphere. Since a GaN-based semiconductor material has a high refractive index of about 2.4, a critical angle is relatively large. Light directed toward a surface of an LED at an angle less than a critical angle is totally reflected and then returned again to the interior of the LED. Such light may be again reflected and then radiated to the outside, but a portion of the light is absorbed into the LED or electrodes and then lost in the form of heat. The roughened surfaces prevent light incident on the LED surface from being returned to the interior of the LED due to the total reflection, and thus, allow the light to be radiated to the outside.
Meanwhile, a technology for employing a patterned sapphire substrate to improve light extraction efficiency has been proposed in “InGaN-based near-ultraviolet and blue-light-emitting diodes with high external quantum efficiency using a patterned sapphire substrate and a mesh electrode,” Japanese Journal of Applied Physics, Vol. 41, 2002, pp. L1431-L143, Dec. 15, 2002.
According to the technical paper, a sapphire substrate is etched to form convex hexagons thereon such that light loss due to total reflection of light between an LED and a substrate can be reduced to thereby improve light extraction efficiency.
However, the light extraction efficiency of LEDs has not yet reached a satisfactory level, and thus, continuous efforts to improve the light extraction efficiency are required.